Now we have for commercial and officinal purposes two varieties of the cultivated plant, the black Mustard (_Sinapis nigra_), and the white Mustard (_Brassica_, or _Sinapis alba_). There is also a plain plant of the hedges, [376] Hedge Mustard (_Sisymbrium officinale_) which is a mere rustic Simple. It is the black Mustard which yields by its seeds the condiment of our tables, and the pungent yellow flour which we employ for the familiar stimulating poultice, or sinapism. This black Mustard is a tall smooth plant, having entire leaves, and smooth seed pods, being now grown for the market on rich alluvial soil chiefly in Lincolnshire and Yorkshire. In common with its kindred plants it gets its name from _mustum_, the "must," or newly fermented grape juice, and _ardens_, burning, because as a condiment, Mustard flour was formerly mixed with home-made wine and sugar. The virtues of black Mustard depend on the acrid volatile oil contained in its seeds. These when unbruised and macerated in boiling water yield only a tasteless mucilage which resides in their skin. But when bruised they develop a very active, pungent, and highly stimulative principle with a powerful penetrating odour which makes the eyes water. From thence is perhaps derived the generic name of the herb _Sinapis_ (_Para tou sinesthai tous hopous_, "because it irritates the eyes"). This active principle contains sulphur abundantly, as is proved by the discoloration of a silver spoon when left in the mustard-pot, the black sulphuret of silver being formed. The chemical basis of black Mustard is "sinnigrin" and its acid myronic. The acridity of its oil is modified in the seeds by combination with another fixed oil of a bland nature which can be readily separated by pressure, then the cake left after the expression of this fixed oil is far more pungent than the seeds. The bland oil expressed from the hulls of the black seeds after the flour has been sifted away, promotes the growth of the hair, and may be used with benefit externally for [377] rheumatism. Whitehead's noted Essence of Mustard is made with spirits of turpentine and rosemary, with which camphor and the farina of black Mustard seed are mixed. This oil is very little affected by frost or the atmosphere; and it is therefore specially prized by clock makers, and for instruments of precision.